1. Field of the Invention
The present invention relates to a transmission apparatus for transmitting digital audio and video signals for application in, for example, digital VCRs that record and reproduce the audio and video signals as digital signals.
2. Description of the Prior Art
Digital VCRs for recording digital audio and video signals to tape are currently being developed as a means of achieving high image and sound quality. Recording digital video signals, however, greatly increases tape consumption because of the extremely large amount of information the digital video signal carries. As a result, tape consumption is reduced in consumer digital VCRs by compressing the video signal before recording. Intra-frame video signal compression, whereby compression is completed within each frame, is used so that the tape can be edited frame by frame.
This intra-frame compression method compresses the video signal so that the compressed data quantity of each frame is the same in each frame. The rotary head is therefore controlled to record the data to tape synchronized to the frame cycle of the video signal. As a result, when a signal is dubbed between plural digital VCRs, the recording-side VCR must synchronize rotation of the rotary head to the frame cycle of the video signal output by the reproducing-side VCR. The reproducing-side VCR must therefore transmit the video signal frame information to the recording-side VCR.
The intra-frame compression method described above was developed for VCRs because compression must be completed within each frame to enable the user to edit the video signal on a per-frame basis. This method also enables both compression and decompression processes to be executed by the VCR using relatively small-scale circuitry. At the same time, however, compression methods intended for broadcasting applications, and not for the convenience of VCR recording/reproducing, are also being developed.
Broadcasting systems only need to sequentially output the compressed signals, and do not need to allow for inserting different data to the signal stream as do digital VCRs enabling editing by the end-user. It is therefore not necessary for broadcasting system compression schemes to complete compression within each video frame. The compression circuitry can also be relatively large because the compression process is only executed at the broadcasting station. Compression schemes used by broadcasting stations can therefore be relatively complex operations spanning plural frames as a means of also reducing the bandwidth of the broadcast signal. The decompression process, however, must still be executed by a consumer device, and the decompression circuitry must therefore be as small as possible. A protocol known as MPEG2 is thus being developed as a video and audio compression method suited to broadcasting applications.
Under the MPEG2 protocol, the video signal in particular is compressed in blocks of plural video frames, and the data quantity of the input video signal varies from frame to frame. Each frame of the compressed video signal in this method is either a frame (I-frame) in which compression is completed within one frame (intra-coded frames), or a frame containing only the difference data between two or more frames (predictive coded frames (P-frames) or bi-directionally predictive coded frames (B-frames)). Because the I-frames are intra-frame compression coded, the compression rate cannot be increased, and the data quantity after compression is significantly greater when compared with B-frames and P-frames. In addition, because the B-frames and P-frames contain only the data describing the difference between that frame and the adjacent frame(s) or the I-frame, the data quantity after compression is very small.
Thus, the data quantity of the video data compressed according to the MPEG2 protocol will vary from frame to frame. The compressed frames are then stored to MPEG2 transport packets having a constant packet length (size) for transmission or recording/reproducing. Each transport packet also contains a packet identifier (PID) used to discriminate whether the data stored to that transport packet is video data or audio data. When the data rate is low, dummy data is written to a transport packet, and a dummy transport packet is transmitted.
It is also possible to multiplex plural programs for transmission using a single transmission channel because the data rate of the MPEG2-compressed audio/video data is extremely low compared with the original data rate. Even in multiplexed transmissions, however, it is possible to determine to which program the audio/video data in each transport packet belongs by reading the PID contained in each transport packet.
Each transport packet is then input to a transport packet decoder, and the transport packet decoder selects and stores to a buffer memory the transport packets storing the audio/video data for the program to be reproduced. Note that timing information is also added to each transport packet. This timing information is generated on the transmission side based on a 27-MHz clock (the decode clock), and added to the transport packets.
The decode clock is generated on the receiver side based on the timing information added to a known position in each transport packet. The receiving apparatus comprises a 27-MHz oscillator and a counter for counting 27 MHz, detects the difference between the received timing information and the counter value of the receiver-side 27-MHz counter when a transport packet containing timing information is received, and changes the oscillation frequency of the receiver-side 27-MHz oscillator to minimize this difference. Therefore, to correctly reconstruct the decode clock on the receiver side, the timing at which each transport packet is received from the transmission medium must match the timing at which the transport packet was generated by the transmission apparatus. The compressed audio/video data is then decompressed at this reconstructed decode clock by the receiving apparatus to reconstruct an analog signal.
Digital VCRs and devices for recording or broadcasting audio/video data compressed according to MPEG protocols are currently available. Transmission apparatuses for transmitting video and audio data are now needed to enable digital signal dubbing between two digital VCRs, or to receive and record MPEG2-compressed audio/video data to a digital VCR.
The Institute of Electrical and Electronic Engineers, Inc. (IEEE) is currently considering a next-generation high speed serial bus protocol, IEEE P1394 (see "High Performance Serial Bus"). It is possible by means of the IEEE P1394 protocol to transmit isochronous communications data, i.e., real-time data such as video and audio signal data, by isochronous communications using synchronization packets. P1394 enables constant data rate communications by always sending/receiving one isochronous packet every cycle (which is approximately 125 .mu.sec). The clock and cycle period used by the P1394 protocol have no synchronization relationship to the signal source clock. The data transmitted each cycle using the P1394 protocol is one isochronous packet, which can be transmitted at any particular timing within the cycle. The timing at which a packet is transmitted within the cycle is determined each cycle at the convenience of the P1394 bus (i.e., based on P1394 bus activity).
When transmitting audio/video data compressed by the intra-frame compression scheme used in digital VCRs, frame synchronization between the transmitting and receiving sides is necessary. The clock of the P1394 bus used as the transmission medium, however, has no synchronization relationship to the transmitted video signal frame cycle. In addition, the audio/video data is divided among plural transport packets for transmission, but the timing at which each packet is transmitted is determined at the convenience of the P1394 bus. It is therefore not possible, when transmitting intra-frame compression coded data using the P1394 bus, to obtain correct frame synchronization between the transmitting and receiving sides.
Furthermore, when MPEG2 data is transmitted using a transmission medium, such as the P1394 bus, whereby data is transmitted based on a transmission clock or cycle with no synchronization relationship to the signal source, it is not possible for the receiving apparatus to receive the transport packets at the same timing as the transport packets were output by the transmission-side signal source. As a result, the 27-MHz decode clock cannot be correctly reconstructed by the receiving apparatus.